The heat stable enterotoxin released by some strains of pathogenic strains of Escherichia coli binds to a glycoprotein on the brush border surface of intestinal epithelial cells, activating guanylate cyclase and causing a severe watery diarrhea. This peptide has been shown to have core amino acid sequence homology with peptides released by other bacteria and by eukaryotic organisms. Such strains of E. Coli are a major cause of infectious diarrhea. The long goal of this proposal is to understand the nature of the ligand-receptor protein interaction by use of biochemical and molecular approaches. The specific objectives of experiments described herein will be: (1) to identify the putative gut hormone or peptide homologous to the family of heat-stable, bacterial enterotoxins and (2) to determine the primary structure of the brush border intestinal receptor of E. Coli heat stable enterotoxin by sequence analysis of protein and cDNA.